Absolute strain measurement using fiber Bragg gratings

Exposing ultraviolet light changes the refractive index of the core of germanium-doped silica optical fiber. This effect can be used to produce a periodic refractive index change along the core of the fiber, which results in a fiber Bragg grating. The fiber Bragg grating shows strong reflection at a certain wavelength determined by the round-trip optical path-length between successive periods.;The growth of the index change, the shift of the Bragg wavelength and the broadening of the spectral bandwidth of a fiber Bragg grating are simultaneously measured as a function of UV illuminating time. The results of these measurements are used to make proper fiber Bragg gratings for sensor applications. With a frequency doubled argon ion laser of 130 mW total power, the growth of the index change is measured to follow a power law in time with a power coefficient of 0.275.;The Bragg wavelength of a fiber grating is shifted by the variation of the refractive index and the period of the grating. If a measurand alters any of them, it can be measured by interrogating the shifted Bragg wavelength. Since the measurand information is encoded into a wavelength, in principle, the fiber grating can measure absolute measurands. A cost effective practical sensor system, using a stretchable fiber as a tunable filter, is designed and tested. The filter grating is tuned to be matched the Bragg wavelength of the sensor grating through a home-made closed-loop controller. To overcome the accuracy problems associated with the nonlinearity and hysteresis of a tuning device for the filter grating, a self-sustaining fiber string-oscillator is made and tested. The fiber piece containing the filter grating is activated by a piezo actuator to oscillate in resonance frequency. Since the resonance frequency of a string depends only on the strain applied to it, the strain on the filter grating can be measured precisely and absolutely excepting temperature dependency.;With the stretchable fiber Bragg grating and with the string-oscillator, strain and temperature variations on the sensor fiber grating are measured. The square of the frequency is measured to be linear with the variation of strain on the sensor grating within 0.4% of its whole range,...